Switching units of this type are frequently employed in circuits for controlling installations or machines in order to perform on-off control of all or part of their functions.
Potential applications are numerous. In one example, the key is secured to the inner face of the door of a cubicle which is intended to remain closed when the electric equipments placed within said cubicle are in the energized state. When the door is closed, the key is inserted within the head of the unit, which is secured to the cubicle itself. Upon opening of the door, the key automatically moves apart from the unit, thus automatically deenergizing the equipment. In another application, the key is rigidly fixed to the gate of an industrial elevator said gate needing to be closed prior to startup, for obvious safety reasons. frequently encountered, the operation of the switch is inhibited or locked in response to the appearance or disappearance of certain supplementary conditions. For example, in the case of an elevator, when the elevator is moving between two floors, the key is locked within the head of the unit, thus mechanically preventing opening of the gate and deenergization of the elevator motor.
In a known switch of this type as described in an article by B. Kirschbaum in the review entitled Elektro Anzeiger, Nr. 9 (1985), a large-sized casing is designed to accommodate a device for controlling the switch by means of a key as well as an electromagnet for selectively immobilizing the key as a function of a predetermined operating parameter.
The advantage of this arrangement lies in the fact that the abnormal forces which are liable to arise at the time of any fraudulent attempt to open the casing are transferred to the key which constitutes a high-strength component. However, this arrangement takes up considerable space. At the same time, the length of the key becomes so substantial that it is hardly possible to contemplate movement of introduction of said key along a nonrectilinear path of travel, e.g. a circular path if the key is secured to the door, unless the key is mounted on an orientable support of relatively complicated design. Furthermore, this device does not as a practical matter allow controlling the locking of the key in the head by means of an element other than the internal electromagnet. This is a serious drawback because the external parameter to be taken into account is liable not to be directly or easily available in the form of an electric signal adapted to be fed to the electromagnet.
In a switching device which is also known and disclosed in French patent N.degree. 2 569 303, an electromagnet selectively locks a special transmission rod placed between the switch and the movable component. This feature is comparable to the electromagnet of the previously known embodiment, and has the disadvantage of requiring, if an external condition is to be taken into account, disassembly of the standard switching unit (comprising only the casing on the head) in order to mount an intermediate casing between the casing which accommodates the switch and the head which is intended to receive the key. Furthermore, in the event of fraud such as, for example, an attempt to open the door to which the key is attached, the moveable component may be subjected to excessive efforts which would in turn be liable to cause its destruction. Finally, according to the prior patent, there is no actuation of the switching means by the electromagnet, the electromagnet merely locks the movable component.